Method of metallic component surface modification for electrochemical applications

ABSTRACT

Method for forming a metallic component surface to achieve lower electrical contact resistance. The method comprises modifying a surface chemical composition and creating a micro-textured surface structure of the metallic component that includes small peaks and/or pits. The small peaks and pits have a round or irregular cross-sectional shape with a diameter between 10 nm and 10 microns, a height/depth between 10 nm and 10 microns, and a distribution density between 0.4 million/cm 2  and 5 billion cm 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/594,349 filed on Oct. 7, 2019, which is a continuation of U.S.application Ser. No. 15/130,330 filed on Apr. 15, 2015 (U.S. Pat. No.10,435,782 issued Oct. 8, 2019), which claims priority to U.S.Provisional Application No. 62/147,755 filed Apr. 15, 2015, which isincorporated by reference in their entireties.

FIELD OF THE INVENTION

Embodiments of the invention relate to reducing the surface electricalcontact resistance of metallic components for electrochemicalapplications in, including, but not limited to, fuel cells, electrolysiscells, batteries and capacitors. More specifically, the embodimentsrelate to methods for improving the surface electrical conductance ofmetallic components by surface morphology and chemical compositionmodification.

BACKGROUND

In numerous electrochemical devices, some metallic components need a lowelectrical surface contact resistance with adjacent components. Themetallic components could be electrodes, current collectors, mass (gasor liquid) diffusion layers, or the multifunction components, such asbipolar plates in fuel cells and electrolysis cells. The metalliccomponents must also be corrosion resistant in the operationalenvironment to ensure long term, stable operation. However, mostcorrosion resistance metals and alloys rely on a surface oxide layer forcorrosion protection, and the surface oxide layer is not electricallyconductive.

One example application is the metallic components in electrolysiscells. An electrolyzer is an electrochemical device that performschemical reactions using electrical power. The typical reactions includesplitting water to hydrogen and oxygen, or converting sodium chloride tochlorine gas and sodium hydroxide. A practical electrolyzer includesmultiple cells for sufficient production capacity. These cells areconnected in series using bipolar plates to build a stack with othernecessary hardware such as gas diffusion layers, end plates, cellframes, gaskets, etc. These components are exposed to highelectrochemical potential during operation. Metallic components,typically made of commercial pure titanium, will be continually oxidizedresulting in a thicker surface oxide layer and higher electrical contactresistance. The high surface electrical contact resistance will lead tothe high internal energy loss of the electrolysis cells. The currentsolution is to electrically plate the component with a layer of platinumto ensure low surface electrical contact resistance. The high materialcost of platinum prohibits the broader commercial applications.

Another example application is the current collector of a lithiumbattery. The function of the current collector is to collect electronsto and from electrode active materials to maintain the battery'soperation. It is desired to have low surface electrical contactresistance in high voltage operational conditions. Commercial 3.6Vlithium ion batteries based on the LiPF₆ electrolyte use a high purityaluminum foil as the current collector. It works fine in small to mediumpower batteries. However, both electrical current density and operatingvoltage of high power lithium batteries are much higher thanconventional applications. The internal ohmic loss is high (under highelectrical current) and the aluminum is not stable (under high voltage).More surface conductive and corrosion resistant current collectormaterial is needed for high power lithium batteries.

One further example is the bipolar plates in proton exchange membrane(PEM) fuel cell stacks. The function of the bipolar separate plate isto 1) separate the hydrogen of one cell from the air of the adjacentcell; 2) collect electrons to/from the electrodes; 3) facilitate mass(gas and water) transport in cells and 4) maintain the properoperational temperature of the stack. The operational condition insidethe PEM fuel cell is very corrosive (high potential, in acid solutions).The electrical current density is very high (2-4 A/cm²). Bipolar platesmade of metal foil, such as stainless steel foil, have advantages suchas light weight and higher thermal conductance than that of traditionalgraphite plates. However, it is a challenge to maintain low electricalcontact resistance of a metal plate with a gas diffusion layer (thecomponent in direct contact with bipolar plates), due to the resistivityof the surface oxide layer of corrosion resistant metal plates.

The electrical contact resistance and corrosion resistance requirementsare dependent on the specific applications. For example, the metalliccomponents in PEM electrolysis cell must have a corrosion resistancethat is suitable for over 10 years of stable operation in a slightlyacidic solution at high electrochemical potential (over 2.0 V_(NHE))with electrical contact resistance below 2 mΩ·cm² with a porous titaniumgas diffusion layer. In PEM fuel cells, the metallic bipolar plates musthave sufficient corrosion resistance for stable operation over 6,000hours in an acid solution under an electrical potential over 0.8V_(NHE)at 80-90° C. The surface electrical contact resistance must be below 10mΩ·cm² with a carbon felt gas diffusion layer. The current collector forlithium batteries has to be stable in the organic salt electrolytesolutions (typically containing fluorine) at high potential (up to 5V_(Li/Li+)). The surface electrical contact resistance of the currentcollector with electrode should be below 100 mΩ·cm². The requirementsfor a capacitor current collector are also related to the chemicalsystems (aqueous or organic electrolyte based) and the operationalcurrent density of super-capacitors are at a much high current densitythan that of traditional capacitors. Therefore, lower resistant currentcollectors are desired.

In electrochemical devices, metallic components, as a solid piece, arein contact with other solid components, in most cases, under certaincompression pressure. It is well known that the solid-to-solidelectrical contact is built on the direct contact of high points of thesolid surface. The common way to reduce the electrical contactresistance is to use soft materials, such as gold, silver, tin andcopper, that can be deformed under the pressure to increase the contactarea between solid components. An example of this type of surfacemodification is taught in U.S. Pat. No. 6,685,988, which deposits tin ona metal surface to reduce the electrical contact resistance. However,these soft materials are either too expansive or lack the desiredcorrosion resistance for electrochemical applications.

Various methods have been taught for these applications. U.S. Pat. No.6,379,476 teaches a special stainless steel alloy that has electricalconductive inclusions for PEM fuel cell applications. The surface oxidelayer of the special alloy can protect the stainless steel fromcorrosion in PEM fuel cells, and the electrical conductive inclusionscan maintain the low surface electrical contact resistance. US patentapplication publication no. 2005/0089742 teaches a method to etch offthe surface metallic layer of the alloy similar to the one taught inU.S. Pat. No. 6,379,476 to expose the electrical conductive inclusionsas the electrical contact point.

U.S. Pat. No. 6,723,462 teaches a special chromium-nickel austeniticalloy that can form a more electrical conductive surface oxide thanregular stainless steel, such as commercial available 316L. It can beused as the bipolar plates for PEM fuel cells.

U.S. Pat. No. 5,098,485 teaches a method to convert the electricalinsulating, native poorly conductive surface oxide layer of metalliccomponents to more electrically conductive surface for capacitorapplications.

WO 2007/013508 teaches a Ti alloy with precious metal elements. Theprecious metal in the alloy will lead to the formation of a preciousmetal and titanium oxide composite surface layer that has low surfaceelectrical resistance. It can be used as an electrode or bipolar platesin fuel cells.

In addition to the development of special alloys that have lowelectrically resistive surface oxide layer, another widely used approachis to coat the metallic components with corrosion resistant and surfaceelectrical conductive materials, typically precious metals. US patentapplication publication no. 2003/0124427 teaches a method to deposit athin layer of gold on titanium plate by sputtering process. The goldcoated titanium plate is used as the current collector in nonaqueousLi/CF_(x) cells. Platinum coated titanium has been widely used inelectrolyzers. Gold coated stainless steel plates were also used for PEMfuel cell applications. In general, the corrosion resistance andelectrical conductive coating on a low cost substrate material is apractical approach for various applications, as long as the coatingmaterial has reasonable cost, and the coating process is reliable andlow cost.

Neil Aukland reported a group of low surface electrical contactresistance titanium alloys that contain 1-3 atomic % niobium or tantalum(J. Mater. Res., Vol. 19, No. 6, pp. 1723-1729, June 2004). This alloycan form a semi-conductive niobium doped titanium oxide (or tantalumdoped titanium oxide) surface layer that leads to lower surfaceelectrical contact resistance than pure titanium. This method has theadvantage of low metal cost by eliminating precious metal. However, itssurface electrical contact resistance is still too high for PEM fuelcell applications.

Therefore, it is desired to have metallic components that have highcorrosion resistance and low surface electrical contact resistance forelectrochemical applications. It could be either a special alloy or acoating on a metallic components. Methods for the alloy and the coatingproduction and surface treatment are also needed to produce low cost,durable metallic components for electrochemical applications.

SUMMARY OF THE INVENTION

Disclosed embodiments provide a method for forming a metallic componentsurface to achieve lower electrical contact resistance. The methodcomprises modifying a surface of the metallic component to form smallpeaks and pits that are capable of serving as high-pressure contactpoints with other solid components of a device of which the metalliccomponent is a part. The small peaks and pits have a diameter between 10nm and 10 microns, a height between 10 nm and 10 microns, and adistribution density between 0.4 million/cm² and 5 billion cm². Themethod also comprises modifying the surface oxide layer composition ofTi alloy or Ti alloy coating to obtain the more electrical conductivesurface oxide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the set up for the surface electrical contact resistancemeasurement.

FIG. 2A is the relation of electrical conductance of niobium dopedtitanium oxide with the concentration of niobium.

FIG. 2B is the Phase Diagram of Ti—Nb alloy.

FIG. 3 is a schematic cross-sectional view of a metallic plate surfacestructure including round top peaks.

FIG. 4 is a schematic cross-sectional view of a metallic plate surfacestructure including sharp top peaks.

FIG. 5 is a schematic cross-sectional view of a metallic plate surfacestructure including valleys and pits.

FIG. 6 is a SEM picture of a titanium foil surface at a 52° viewingangle, according to an embodiment. The surface includes many small peaksthat have sharp tops.

FIG. 7 is a SEM picture of a titanium coated 316L stainless steel foilsurface at a 52° viewing angle, according to an embodiment. The surfaceincludes many small round titanium peaks.

FIG. 8 is a SEM picture of a hydrofluoric acid etched titanium coated316L stainless steel foil at 5,000× magnifications, according to anembodiment.

FIG. 9 is a SEM picture of a hydrofluoric acid etched titanium coated316L stainless steel foil at 50,000× magnifications, according to anembodiment.

FIG. 10 is a SEM picture of a sputtered titanium coated 316L stainlesssteel foil, according to an embodiment.

FIG. 11 is a SEM picture of as sputtered Ti2Nb-SS according to anembodiment.

FIG. 12 is a SEM picture of as etched Ti2Nb-SS at ×10,000 magnification,according to an embodiment.

FIG. 13 is a SEM picture of as etched Ti2Nb-SS at ×50,000 magnification,according to an embodiment.

FIG. 14 is a SEM picture of a cathodic arc deposited Ti-2Nb on stainlesssteel surface, according to an embodiment.

FIG. 15 is a SEM picture of an etched cathodic arc deposited Ti-2Nb onstainless steel surface, according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, a plurality of specific details,such as types of materials and dimensions, are set forth in order toprovide a thorough understanding of the preferred embodiments discussedbelow. The details discussed in connection with the preferredembodiments should not be understood to limit the claimed inventions.Furthermore, for ease of understanding, certain method steps aredelineated as separate steps; however, these steps should not beconstrued as necessarily distinct nor order dependent in theirperformance.

An object of the embodiments disclosed herein is to provide a metalsurface structure and treatment process to provide low surfaceelectrical contact resistance and maintain high corrosion resistance.The metal surface morphology modification is used to achieve theobjective. More specifically, the metal surface morphology modificationis to convert the surface to a micro-textured structure that includestremendous amounts of standouts, peaks, spikes, valleys, pits. Themicro-textured structure creates a tremendous amount of high pressurecontacting points and more actual contacting areas that can reduce thesurface electrical contact resistance of the component.

Another object of the embodiments disclosed herein is to provide methodsto modify surface oxide chemical composition of titanium alloy andtitanium alloy coating on metallic component to obtain an electricalconductive oxide surface layer. More specifically, the surface oxide isgrown on the titanium alloy surface by the reaction of titanium alloywith oxygen. Its composition is tailored to have higher niobium ortantalum concentration than that in the alloy to achieve higherelectrical conductance of the oxide surface layer.

As used herein, the symbol “%” refers to atomic % of all metallicelements in the alloy or oxide (metal based concentration), unlessotherwise noted. High valance elements are elements whose cations havethe valance more than +4. Examples of high valance element are niobium,tantalum, vanadium, molybdenum, and tungsten. Low valance elements areelements whose cations have the valance of +4 or less. Examples of lowvalance element are aluminum, zirconium, hafnium, yttrium, iron, nickel,and chromium.

In this description, doped titanium oxide is the mixture of titaniumoxide with other metal oxide. Titanium is the dominating metallicelement in the mixture. The minor metallic elements are called dopants.For example, 5% niobium doped titanium oxide has the formulation ofNb_(0.05)Ti_(0.95)O_(x). The oxygen content x is between 2 and 1, whichis related to the dopant concentration and processing condition of thedoped titanium oxide.

In this description, doped titanium oxide can be also described as theweighted average valance of all metallic elements, including titanium,in the oxide layer. The valance of titanium in the calculation is +4,despite the fact that titanium may also be present in +2 and +3 valancestatus in the doped oxide. The equation of the calculation for oxidewith formulation of A⁺⁶ _(m)B⁺⁵ _(n)C⁺⁴ _(o)D⁺³ _(p)E⁺² _(q)O_(x) is.

Weighted Valance=6m+5n+4o+3p+2q, where m, n, o, p and q are the atomicconcentration of metallic elements and m+n+o+p+q=1. For example, theweighted average valance of Nb_(0.08)Al_(0.02)Ti_(0.9)O_(x) is equal to0.08×5+0.02×3+0.9×4=+4.06. In the same calculation, the weighted averagevalance of pure titanium dioxide (TiO₂) is +4, andNb_(0.05)Ti_(0.95)O_(x) is +4.05.

In this description, doped niobium pentoxide is the mixture of niobiumpentoxide with other metal oxide. Niobium is the dominating metallicelement in the mixture. The minor metallic elements are called dopants.For example, 10% titanium doped niobium pentoxide has the formulation ofTi_(0.2)Nb_(1.8)O_(x). The oxygen content x is between 5 and 4.

In this description, the surface layer composition is determined byauger electron spectroscopy (AES) or X-ray Photoelectron Spectroscopy(XPS) that detects chemical composition in the depth of 5-9 nm from thesurface. Other methods may detect the surface composition with differentdepth range, but are not used in this description.

In this description, some embodiments include the metal surfacemorphology that has stand-outs, spikes, peaks, valleys and pits. Theterm “peaks” is used to describe all surface features that stand out ofthe substrate surface, such as spikes, stand-outs and peaks. The term“pits” is used to describe all surface features that are below thesubstrate surface, such as valleys and pits. The dimension of thesepeaks and pits are characterized as the height and width. The height isthe vertical distance between the top and bottom of peaks. The depth isthe vertical distance between the top and bottom of the pits. The widthfor round shaped peaks and pits is the diameter at the half height (ordepth). Irregularly shaped peaks and pits can be characterized as theclosest sized round shape, or multiple round shapes joined together. Thewidth of narrow sheet shape peaks and pits are characterized by thewidth at the half height (or depth). All dimensions are thestatistically average value.

An advantage of the disclosed methods is that they can produce, at a lowcost, metal components for electrochemical application that need highelectrical conductance and corrosion resistance for long term operation.

FIG. 1 is the fixture used in the disclosed embodiments to measure thesurface electrical contact resistance (CR). Reference numeral 1 is asample of a metallic component under measurement, typically a flat metalplate. Reference numerals 2 and 3 are the contacting elements that arepressed on sample 1. Reference numerals 4 and 5 are electrodes thatpress elements 2 and 3 on sample 1. Reference numeral 6 is a presserthat provides the compression force on the assembly of components 1-5.The typical material of contacting elements 2 and 3 are porous carbonfelt sheets, such as TGP-H-060 Toray paper. Platinum coated titaniummeshes are also used as the contacting elements 2 and 3 for someapplications. The electrodes 4 and 5 are gold coated stainless steel.Reference numeral 7 is a DC power source that provides DC current (I)through the assembly of components 1-5. Reference numeral 8 is avoltmeter that measures the voltage drop between electrode 4 and sample1 (V₁). Reference numeral 9 is another voltmeter that measures thevoltage drop between two electrodes 4 and 5 (V). Reference numeral 10 isan ammeter that measures the current passing through the assembly ofcomponents 1-5. The typical contacting area between sample 1 andcontacting elements 2 and 3 is at the size of 1 square inch and the DCelectrical current is 6.45 A, which makes the electrical current density(i) passing through the contacting elements (2 and 3) and sample 1contacting surface at 1A/cm².

During the measurement, the voltage drops (V and V₁) between electrodes4 and 5, and electrode 4 with sample 1 are measured, at first. Then thevoltage drop (V_(base)) between electrode 4 and 5 is measured withcontact elements 2 and 3 only (without sample 1) as the baseline. Thesurface contact resistance (CR) of contacting element 2 with sample 1 iscalculated as:CR=(V ₁ −V _(base)/2)/iThe through plate resistance (TPR) of the sample is calculated as:TPR=(V−V _(base))/iBecause the bulk resistance of metallic component is very small, the TPRis actually the sum of the surface contact resistance of the sample 1with contacting element 2 (CR₁₋₂) and sample 1 with contacting element 3(CR₁₋₃).TPR=CR₁₋₂+CR₁₋₃

The surface electrical contact resistance measurement is normallyconducted with the compression pressure between 15 psi and 300 psi thatrepresents the pressure range for difference applications. For example,the compression pressure is −15 psi in lithium batteries and capacitors,100-200 psi in PEM fuel cells; and much higher in high pressureelectrolyzers. The pressure is needed to compress all cell componentstogether to achieve the best performance.

Although titanium oxide is an electrical insulator, it can be changed tosemi-conductive by doping a high valance element (M⁺⁵, M⁺⁶ or M⁺⁷, M isa metallic element). The high valance elements include niobium,tantalum, vanadium, molybdenum, and tungsten. The electricalconductivity of the doped titanium oxide is determined by the dopantconcentration. FIG. 2A shows the relation of the electrical conductanceof niobium doped titanium oxide with the concentration of niobium. Itshows that niobium doped titanium oxide reaches the highest bulkconductance (σ_(b)) with the niobium concentration between 5-7%. On theother hand, the grain boundary conductivity (σ_(gb)) is related to thefabrication process. It could reach the highest conductivity at a muchhigher concentration of niobium in titanium oxide, reported by A.Trenczek-Zajac and M. Rekas in Materials Science-Poland, Vol. 24, No. 1,2006.

In addition, if the dopant concentration is too high, the doped titaniumoxide cannot maintain titanium dioxide crystal structure, such asrutile, anatase or brookite structure, the doped titanium oxide willlose its semi-conductive properties, and change back to an insulator. Itwas reported that the non-conductive TiNb₂O₇ phase was found if theniobium concentration is higher than 15%. Therefore, it is desired toobtain the optimized concentration of dopant in titanium oxide. In thecase of niobium doped titanium oxide, the optimized concentration ofniobium is 4-15%.

FIG. 2B is the phase diagram of a titanium-niobium alloy, as in “BinaryAlloy Phase Diagrams”, edited by T. B. Masslski, J. L. Murray, L. H.Bennett and H. Baker and published by American Society of Metals in1986. It shows that the titanium-niobium alloy has a single α-phase attemperature between 400 and 882° C. with the niobium concentration of 0to 2%. At the niobium concentration above 37%, the alloy will have thesingle β-phase at 400° C. At the niobium concentration between 2 and37%, the alloy will be the mixture of low niobium concentration α-phaseand the niobium concentration β-phase at 400° C. Although there is noreported data of a titanium-niobium phase diagram at temperatures below400° C., it is expected that the phase structure will be similar to thatat 400° C., except the niobium concentration range for α- and β-phasescould be slightly different than that at 400° C. For example, themaximum niobium concentration in α-phase could be in the range of 1-3%,and the minimum niobium concentration in β-phase could be in the rangeof 35-40% at room temperature.

It is common knowledge that the actual phase structure of the alloy atroom temperature is also related to the processing conditions thatdetermine the phase transfer reaction kinetics. In some cases, it isalso possible to obtain a single phase structure of the alloy at roomtemperature at the two-phase structure concentration range.

The doped titanium oxide can be prepared as the oxide ceramic material.It can also grow on a titanium alloy surface in the form of thin surfaceoxide layer.

The composition of the surface oxide layer is highly related to thesurface composition of the metallic substrate. Typically, the metallicsurface with a higher niobium concentration will form the titanium oxidesurface layer with a higher niobium dopant concentration. For example,the low niobium content α-phase alloy surface will form a lowconcentration niobium doped titanium oxide surface layer. The highniobium content β-phase alloy will form a high concentration niobiumdoped titanium oxide surface layer.

In the case of the two-phase Ti—Nb alloy, the composition of the surfaceoxide is not uniform. The surface oxide layer on the low niobium contentα-metallic phase will have a low concentration niobium doped titaniumoxide, and the surface oxide layer on the high niobium contentβ-metallic phase will have a high concentration of niobium mixed withtitanium oxide. For example, the surface oxide on α-phase with 2%niobium normally have the composition close to 2% niobium doped titaniumoxide, and the surface oxide on β-phase with 40% niobium normally havethe composition close to 40% niobium oxide and titanium oxide mixture.

As discussed above, the surface oxide layer on the low niobium contentα-phase is semi-conductive, but on the high niobium content (up to 40%)β-phase could be non-conductive because the too high niobium dopantconcentration changes the crystal structure of the oxide and results innon-electrical conductive phases. Therefore, it has to control theniobium concentration in the alloy to avoid the resistive phase in thesurface oxide layer.

Other Ti alloys, such as Ti—Ta, Ti—V, Ti—Mo and Ti—W have similarlimitations in maintaining high dopant element concentration in theα-phase of Ti alloy. Its formulation has to be carefully controlled toavoid resistive phases in the surface oxide layer.

These inherent properties of the titanium alloy and doped titaniumoxides are the reasons that limit further lower surface electricalcontact resistance in Neil Aukland's work. They can only obtain up to 3%concentration of niobium or tantalum in the titanium alloy to avoid theresistive phase in the surface oxide layer, which cannot providesufficient dopants in the doped titanium oxide surface layer to achievethe low surface contact resistance for the targeted PEM fuel cellapplication.

Another type of stable semi-conductive oxide is low element dopedniobium pentoxide. For example, +4 valance titanium doped in niobiumpentoxide forces some Nb′ ions convert to Nb⁺⁴ status, which makes theniobium pentoxide more electrically conductive. Similar with dopedtitanium oxide semiconductor, the doped niobium oxide has to keep thepentoxide crystal structure that has the metal ion to oxygen ratio of2:5. The most common dopant is titanium oxide. Other low valancemetallic elements, such as zirconium, yttrium, hafnium and scandiumoxide can also be used.

It is the objective of disclosed embodiments to overcome theselimitations by methods that can modify the chemical composition of thesurface oxide layer to achieve lower surface electrical contactresistance.

FIG. 3 is the schematic cross sectional drawing of a metal surface withpeaks on the surface. The peaks have a round shape at the top. Referencenumeral 11 is the body of the metallic component as the substrate.Reference numeral 12 is the peaks on the surface of metallic components.The peaks 12 could have the same material as the metallic body 11, ordifferent materials.

FIG. 4 is the schematic cross sectional drawing of the metal surfacewith peaks on the surface. The peaks have a sharp, spiky shape at thetop. Reference numeral 21 is the body of the metallic component.Reference numeral 22 are the peaks on the surface of metalliccomponents. The peaks 22 could have the same material as the metallicbody 21, or different materials.

In one embodiment disclosed herein, the surface of a metallic componentis modified to have small peaks. The small peaks create manyhigh-pressure contacting points and actual contacting areas with othersolid components. In some embodiments, the small peaks will have elasticor plastic deformation under the high contacting pressure that result ina high surface contact area with the components in contact, and the lowelectrical contact resistance. In these embodiments, the small peaksbehave like the soft contact material in previous art. But the softnessis achieved by the small dimension, instead of the material's inherentproperties, like gold and tin. In some embodiments, the peaks can pinthrough the high resistive surface layer of the component in contact,with high-pressure on the contacting points to establish a lowresistance contact

The peaks could have a round or irregular shape with the diameterbetween 10 nm to 10 microns. They can point straight up from thesurface, or point sideways. It also could have a shape of thin sheetwith the thickness between 5 nm to 5 microns. The height of the peaks isbetween 10 nm to 10 microns. The distribution density (the total amountof peaks in one cm² area) of the peaks is between 0.5 million/cm² and 5billion/cm². The top of the peaks could have a round shape asillustrated in FIG. 3 , or sharp shape as illustrated in FIG. 4 . Insome embodiments, the peaks have a diameter or width in a range of 5nm-10 nm, 5 nm-100 nm, 50 nm-500 nm, 10 nm-20 μm, 5 nm-0.5 μm, 20 nm-1μm, 100 nm-0.9 μm, 20 nm-5 μm, 100 nm-2 μm, 0.5 μm-5 μm, or 1 μm-10 μm.In some embodiments, the peaks may have a height in a range of 10 nm-100nm, 50 nm-500 nm, 10 nm-10 μm, 50 nm-0.5 μm, 20 nm-1 μm, 100 nm-0.9 μm,20 nm-5 μm, 100 nm-10 μm, 0.5 μm-5 μm, or 1 μm-10 μm. In someembodiments, the distribution density of the peaks is in a range of0.5-100 million/cm², 1-200 million/cm², 5-200 million/cm², 5-500million/cm², 10 million-1 billion/cm², 20 million-2 billion/cm², 50million-5 billion/cm², 100 million-5 billion/cm².

In one embodiment, the metal substrate 11 surface is coated with anothermaterial (not shown in FIG. 3 ) for better corrosion resistance andelectrical conductance. The peaks 12 could be the same or differentmaterial as the coating material. The thickness of this coating materialis between 5 nm to 5 μm. In some embodiments, the thickness of thecoating is between 5-20 nm, 10-50 nm, 20-200 nm, 50-500 nm, 0.1-0.5 μm,0.2-1 μm, 0.5-2 μm, 1-5 μm, and 2-5 μm.

In one embodiment, the metal substrate 11 surface is modified to have amicro-textured surface structure, then the micro-textured metalsubstrate is coated with another material (not shown in FIG. 3 ) forbetter corrosion resistance and electrical conductance. The thickness ofthis coating material is between 5 nm to 5 μm. In some embodiments, thethickness of the coating is between 5-20 nm, 10-50 nm, 20-200 nm, 50-500nm, 0.1-0.5 μm, 0.2-1 μm, 0.5-2 μm, 1-5 μm, and 2-5 μm.

In one embodiment, the surface of the metallic component is modified tohave small pits on the surface. These pits have sharp edges that performas high pressure points to reduce electrical contact resistance whenthey are in contact with other components. The pits can have round orirregular shapes with a diameter between 5 nm to 20 microns. The pitscan also have narrow gap shapes that have a width between 10 nm to 10microns. The depth of the pits is 50 nm to 10 microns. The distributiondensity of the pits is between 0.5 million/cm² and 5 billion/cm². Insome embodiments, the pits have a diameter or width in a range of 5nm-10 nm, 5 nm-100 nm, 50 nm-500 nm, 10 nm-20 μm, 5 nm-0.5 μm, 20 nm-1μm, 100 nm-0.9 μm, 20 nm-5 μm, 100 nm-2 μm, 0.5 μm-5 μm, 1 μm-10 μm or 5μm-20 μm. In some embodiments, the pits have a depth in a range of 50nm-500 nm, 50 nm-10 μm, 70 nm-0.5 μm, 70 nm-1 μm, 100 nm-0.9 μm, 100nm-5 μm, 100 nm-10 μm, 0.5 μm-5 μm, or 1 μm-10 μm. In some embodiments,the distribution density of the pits is in a range of 0.5-100million/cm², 1-200 million/cm², 5-500 million/cm², 5-500 million/cm², 10million-1 billion/cm², 20 million-2 billion/cm², 50 million-5billion/cm², 100 million-5 billion/cm².

FIG. 5 is the schematic cross section drawing of the metal surface withpits 32 on the surface of a metallic component 31.

In one embodiment, the metallic component surface has both peaks andpits as illustrated in FIGS. 3-5 . The dimensions and distribution ofpeaks and pits are the same as those described in the first embodimentdescribed above.

In one embodiment, a low alloy element concentration, single phasetitanium alloy substrate or the titanium alloy coating is prepared.Then, the alloy surface is chemically etched to remove the originalsurface oxide layer. With the proper control of the etching condition,the etched surface has a higher alloy element concentration than thebulk of the alloy, due to the different etching rate of titanium and thealloy element. The difference in the etching rate between titanium andalloy elements is determined by the etchant concentration, etching time,etching temperature. At low etchant concentration and short etchingtime, difference of titanium etching and alloy element etching may betoo small to enrich alloy elements in the surface oxide layer. In orderto obtain the alloy element enriched surface layer, the etching shouldbe performed in the condition of sufficient etchant concentration andetching time. At room temperature, the etched surface with the highalloy element concretion is still in the original single phasestructure, due to the kinetic limitation of phase transfer at roomtemperature. A thin oxide surface layer, then, naturally and rapidlyforms on the etched titanium alloy surface with higher alloy element(dopant) concentration. The oxide surface layer on the etched titaniumalloy surface has high dopant concentration than that formed on theoriginal titanium alloy surface. The higher dopant concentration canincrease the electrically conductive surface oxide layer and reduce thesurface electrical contact resistance of the alloy with othercomponents.

In one embodiment, a low alloy element concentration, single phasetitanium alloy or the alloy coating is prepared. Then, the alloy surfaceis heated up to high temperature. The alloy elements could diffuse andaccumulate on the surface oxide layer resulting in a high dopantconcentration surface oxide layer. The heat treatment temperature isbetween 100-800° C. for 10 seconds to 1 hours. In some embodiments, theoxidization temperature is between 100-300° C., 250-400° C., 400-600°C., 550-700° C., 500-800° C. In some embodiments, the heat treatmenttime is 10 seconds to 1 minutes, 30 seconds to 5 minutes, 1 minutes to20 minutes, 10 minutes to 1 hour. In one embodiment, the alloy elementis niobium or tantalum.

The alloy element is a high valance metallic element, including Nb, Ta,V, Mo, W. The concentration of the high valance element in the titaniumalloy is between 0.01 and 3%, in a binary Ti alloy to maintain thesingle α-phase structure. In some embodiments, the concentration of thehigh valance elements in the titanium is in a range of 0.01-0.1%,0.01-0.5%, 0.1-1%, 0.2-1%, 0.5-1%, 0.2-2.5%, 0.5-3%, 1-2% and 1-3%. Insome embodiments with the titanium alloy containing more than one alloyelement, high valance elements must be included, and the composition ofthe alloy has to be selected to maintain the single phase structure, andthe total alloy elements' concentration should be less than 30%. In someembodiments, the total alloy element concentration is in the rage of0.1-1%, 0.5-2%, 0.5-5%, 1-8%, 1-10%, 2-15%, 5-20%, 1-30%, 5-30% and10-30%.

The concentration of the high valance element in the surface oxide layerof the binary alloy is 3-25%. In some embodiments, the high valanceelement concentration in the surface oxide layer is in the rage of 3-5%,3-8%, 4-10%, 5-10%, 5-15%, 5-15%, 7-20%, 7-25%, 10-25% and 15-25%. Insome embodiments of the etched surface oxide layer containing more thanone alloy element, the concentration of the high valance (M⁺⁵, M⁺⁶, M⁺⁷)elements must be high enough to compensate for the low valance (M⁺³,M⁺², M⁺⁷) elements, and maintain the weighted average valance ofmetallic elements in the surface oxide in the range of +4.02 to +4.25.In some embodiments, the weighted average valance is in the range of+4.02 to +4.05, +4.02 to +4.10, +4.03 to +4.20, +4.05 to +4.10, +4.05 to+4.15, +4.05 to +4.20, +4.07 to +4.15, +4.10 to +4.20, +4.15 to +4.20and +4.15 to +4.25.

The composition of the surface oxide layer on the titanium alloy surfacecan be tailored by the composition of the initial titanium alloy, theetching method, etchant composition and etching conditions (temperature,concentration, etching time, etc.).

The etching method includes the wet chemical etching, electrochemicaletching, plasma enhanced reactive ion etching, vapor etching or otherdry etching processes. In plasma etching process, the etchant isfluorine or chlorine radicals that is be generated from fluorine orchlorine containing compounds under plasma. The radical concentration issignificant higher at high plasma power. The etched titanium alloysurface will naturally grow a new oxide surface layer, as soon as it isin contact with oxygen or water, due to the high activity of titaniumwith oxygen.

The electrochemical stability of the semi-conductive doped titaniumoxide surface layer on the titanium alloy can be enhanced by increasingthe surface oxide thickness with controlled surface oxidizationprocesses, such as thermal oxidization, plasma oxidization, wet chemicaloxidization and anodizing. The thicker oxide layer increases the initialsurface electrical contact resistance of the alloy, but makes it morestable in the operational environment of electrochemical devices. Thethermal oxidation temperature is between 100-800° C. In someembodiments, it is in the range of 100-200° C., 100-400° C., 200-450°C., 200-500° C., 300-500° C., 400-600° C. and 500-800° C.

In another embodiment, a low cost metal is used as the substratematerial of the metallic components in electrochemical devices. Atitanium alloy is coated on the low cost metallic substrate surface toimprove the corrosion resistance and reduce the surface electricalcontact resistance of the substrate metal. The titanium alloy containshigh valance alloy elements and has a single phase structure after it iscoated on the substrate surface. The phase could be α-phase, β-phase oramorphous phase. For the titanium alloy coating material that is not inthe single phase at room temperature, the coating process has to rapidlyquench the coating alloy from high temperature to freeze the hightemperature single phase to low temperature during the coating process.The suitable coating process includes thermal spray and cathodic arcdeposition. The titanium alloy coating surface will form asemi-conductive surface oxide surface layer that has low surfaceelectrical contact resistance and enhance the corrosion resistance ofthe low cost metallic substrate material. The alloy element is a highvalance metallic element, including Nb, Ta, V, Mo, W. The concentrationof the high valance element in the titanium alloy is between 0.01 and20%, in a binary Ti alloy to maintain the single phase coating. In thecase of titanium alloy containing more than one alloy element, highvalance elements must be included, and the total alloy elementconcentration is between 0.01 and 40%. In some embodiments, the totalalloy elements concentration is in the range of 0.01-0.2%, 0.1-1%,0.5-2%, 0.5-5%, 1-8%, 1-10%, 1-30%, 1-40%, 2-15%, 5-20%, 5-30%, 5-40%,15-30%, 10-30%, 10-40%.

In some embodiments, the surface electrical contact resistance of thetitanium alloy coated surface could be further reduced by chemicallyetching to tailor the surface composition in the way that the surfaceoxide layer has the optimized composition and morphology for the lowestsurface electrical contact resistance. The etched titanium alloy coatingsurface forms the doped titanium oxide surface layer. For the coating ofthe binary titanium alloy with the high valance alloy element, thedopant concentration in the surface oxide layer is 3-15%. In someembodiments, the high valance element concentration in the surface oxidelayer is in the rage of 3-5%, 3-8%, 4-10%, 5-10% and 5-15%. For thecoating of multi elements alloy, the concentration of the high valance(M⁺⁵, M⁺⁶, M⁺⁷) elements in the surface oxide layer must be high enoughto compensate for the low valance (M⁺³, M⁺², M⁺¹) elements in thesurface oxide layer, and maintain the weighted average valance ofmetallic elements in the surface oxide in the range of +4.02 to +4.25.In some embodiments, the weighted average valance is in the range of+4.02 to +4.05, +4.02 to +4.10, +4.03 to +4.20, +4.05 to +4.10, +4.05 to+4.15, +4.05 to +4.20, +4.07 to +4.15, +4.10 to +4.20, +4.15 to +4.20and +4.15 to +4.25.

The electrochemical stability of the semi-conductive doped titaniumoxide surface layer on the titanium alloy coating can be enhanced byincrease the surface oxide thickness with controlled surface oxidizationprocesses, such as thermal oxidization, plasma oxidization, wet chemicaloxidization and anodizing. The thicker oxide layer increases the initialsurface electrical contact resistance of the alloy, but makes it morestable in operation environment of electrochemical devices. The thermaloxidation temperature is between 100-800° C. In some embodiments, it isin the range of 100-200° C., 100-400° C., 200-450° C., 200-500° C.,300-500° C., 400-600° C. and 500-800° C.

The low cost substrate material may be one of iron, chromium, nickel,cobalt, manganese, aluminum and alloys containing these elements.Examples of the substrate are commercial pure titanium, 316L stainlesssteel and 6061 aluminum alloy.

The Ti alloy coating thickness is between 0.01 μm and 100 μm, preferredbetween 0.05 μm and 5 μm. In some embodiment, the thickness of thetitanium alloy coating is in the rage of 0.01 μm-0.05 μm, 0.01 μm-0.1μm, 0.05 μm-0.2 μm, 0.05 μm-0.5 μm, 0.1 μm-0.5 μm, 0.1 μm-1 μm, 0.5 μm-3μm, 0.5 μm-5 μm, 0.5 μm-10 μm, 1 μm-10 μm, 0.5 μm-50 μm, 1 μm-50 μm, 5μm-50 μm, 5 μm-100 μm and 10 μm-100 μm.

The deposition method for the titanium alloy coating includes physicalvapor deposition (PVD) and a thermal spray process.

The composition of the etched titanium alloy coating surface layer canbe tailored by the composition of the initial titanium alloy, theetching method, etchant composition and etching conditions (temperature,concentration, etching time, etc.) and thermal oxidization. The etchingmethod includes wet chemical etching, electrochemical etching, plasmaenhanced reactive ion etching, vapor etching or other dry etchingprocesses.

The surface electrical contact resistance of the coated plates can befurther reduced by the modification of the surface morphology to the waythe surface is constructed with the small peaks and pits. The dimensionsand distribution of peaks and pits are same as those described above.

The peaks and pits on the micro-structured surface can be formed byeither removing materials from metallic substrate surface or addingmaterials onto metallic substrate surface, such as by chemically etchingthe Ti alloy coated metallic substrate, or depositing the Ti alloycoating on the substrate surface. The treatment process to obtain themicro-textured surface structure also includes cold rolling, sandblasting, water jet blasting, laser engraving and depositing othermaterials to build the peaks on the surface. The deposition method couldbe PVD, thermal spray, electrical plating and other additive manufactureprocesses.

Typical metallic plates are produced by a multi-step rolling and stressrelease heat treatment. As the result, the grain of the polycrystallinestructure is fairly large, such as 10-100 μm in diameter. The etching ofthis type of material will only obtain large size (>10 μm) peaks andpits that cannot sufficiently reduce the metal plate surface electricalcontact resistance. In one embodiment, the grain size of the metalliccomponent or its surface layer is reduced to <10 μm, or even <1 μm.Then, a chemical etching method can be used to remove some grains fromthe surface to obtain the desired micro-texture surface structure,described above, having low electrical contact resistance.

The grain size reduction can be achieved by a rapid melting andsolidification of the metal substrate surface to obtain the amorphous ornano-crystalized structure of the surface layer. The thickness of there-crystalized layer is 0.5-20 μm. A heat treatment step is then used toobtain the designated grain size. In general, a higher heat treatmenttemperature and a longer treatment time will lead to larger grain size.For the typical stainless steel and titanium alloys, the heat treatmenttemperature is between 800-1200° C., and the heat treatment time is0.5-5 hours.

The grain size of the surface layer can also be obtained by deposit acoating material at temperature below 800° C. The low depositiontemperature will ensure the small grain size of the coating layer. Inone embodiment, a physical vapor deposition process is used to depositthe coating material on the metal substrate surface. The typical grainsize of the coating layer is less than 1 μm.

In one embodiment, the metallic substrate is produced by a rapidsolidification process, such as melt spinning, to obtain the amorphousor nano-crystalized structure. A heat treatment step is then used toobtain the designated grain size of the metal substrate. In general, ahigher heat treatment temperature and a longer treatment time will leadto larger grain size. For typical stainless steel and titanium alloys,the heat treatment temperature is between 800-1200° C., and the heattreatment time is 0.5-5 hours.

The electrochemical stability of the semi-conductive doped titaniumoxide surface layer on the titanium alloy coating can be enhanced byincreasing the surface oxide thickness with controlled surfaceoxidization processes, such as thermal oxidization, plasma oxidization,wet chemical oxidization and anodizing. The thicker oxide layerincreases the initial surface electrical contact resistance of thealloy, but it makes the oxide layer more stable in the operationalenvironment of electrochemical devices. The thermal oxidationtemperature is between 100-800° C. In some embodiments, it is in therange of 100-200° C., 100-400° C., 200-450° C., 200-500° C., 300-500°C., 400-600° C. and 500-800° C.

In one embodiment, a niobium alloy is used as the coating material on alow cost metal substrate, such as stainless steel or aluminum. Theniobium alloy contains low valance alloy elements, such as titanium,zirconium, hafnium, yttrium, scandium, aluminum. The concentration ofthese alloy elements in a niobium alloy is between 1 and 25%. After theniobium alloy is coated on the low cost metal substrate, a doped niobiumpentoxide surface layer is formed on the surface. The doped niobiumpentoxide surface layer can be formed by natural oxidization in air atroom temperature, or enhanced oxidization by anodizing, wet chemistryoxidization with oxidizers such as nitric acid or hydrogen peroxide,thermal oxidization or plasma oxidization. The semi-conductive dopedniobium pentoxide surface layer can keep the low electrical surfacecontact resistance of metallic components for electrochemicalapplications. In some embodiments, the dopant element concentration inthe niobium alloy coating material is in the rage of 1-5%, 1-10%, 1-15%,2-15%, 2-20%, 3-30%, 5-20%, 5-25%, and 10-25%. The thermal oxidationtemperature is between 100-600° C. In some embodiments, it is in therange of 100-200° C., 100-400° C., 200-450° C., 200-500° C., 300-500°C., 400-600° C. and 500-800° C.

The niobium alloy coating layer is between 0.01-10 microns thick. Insome embodiments, the thickness of the niobium alloy coating is in therage of 0.01 μm-0.05 μm, 0.01 μm-0.1 μm, 0.05 μm-0.2 μm, 0.05 μm-0.5 μm,0.1 μm-0.5 μm, 0.1 μm-1 μm, 0.5-3 μm, 0.5-5 μm, 0.5-10 μm and 1-10 μm.

Example 1

A commercial pure titanium foil was selected for the surfacemodification to achieve the low electrical surface contact resistance. A3 cm×5 cm titanium foil was soaked in 50 ml 1.5 wt % hydrofluoric acidsolution for about 2 hours at room temperature. At first, thehydrofluoric acid etched off the native oxide surface layer of titanium.Then, it continually etched the metallic portion of titanium foil.During the etching process, hydrofluoric acid was consumed; itsconcentration was continually decreased and the etching rate of titaniumin the depleted hydrofluoric acid solution was reduced.

The etching process converted the morphology of the flat titaniumsurface to small peaks. FIG. 6 is a SEM picture of the etched titaniumsurface at a 52° viewing angle. It includes large amount of small peaksthat have a sharp top. Some of the peaks are in a thin sheet shape thathas the width between 50-100 nm. Some peaks has the diameter of 50-100nm. The height of the peaks is between 50 nm and 1 micron. Thedistribution density of the peaks is estimated to be close to 1billion/cm².

In order to stabilize the surface oxide layer, the etched titanium foilwas thermally oxidized in air at 450° C. for 1 hour. The through plateresistance (TPR) of the titanium foil was measured with two pieces ofcarbon felt (TGP-H-060 Toray Paper). Table 1 summarizes the TPR of theetched titanium foil and the TPR of the original titanium foil after450° C. thermal oxidization. The table shows that the TPR of the etchedtitanium is 18.1 mΩ·cm² at 150 psi. In comparison, the original titaniumfoil has a TPR of 1.04 Ω·cm² at 150 psi, which is about 58 times higherthan the titanium foil with the modified surface morphology by etching.

TABLE 1 Electrical through plate resistance of original titanium andetched titanium after 450° C. thermally oxidized Through PlateResistance (mΩ · cm²) Original Original Ti Etched Ti 35 1953.05 62.28 511840.82 50.51 91 1153.42 29.00 150 1038.69 18.10 201 960.69 14.25 251892.13 11.87 300 836.81 10.31

The etched titanium surface was superhydrophilic. The foil was muchsofter and more flexible than the original cold rolled foil due to theremoval of the high stress surface layer. The superhydrophilic surfaceproperty has additional advantages for water management in fuel cell andelectrolyzer stacks. The high flexibility of the foil may have thebenefit of being more easily conformed in the electrolyzer stacks forthe optimized stack sealing and integration.

Example 2

A 316L stainless steel foil was used as the substrate. Commercial puretitanium was deposited on the surface by a cathodic arc process(CA-Ti-SS). Cathodic arc is an industrial coating process that can melttitanium target surface and blast melted titanium droplets to thesubstrate, along with titanium vapor. The titanium vapor and dropletsare deposited on the stainless steel substrate surface. The vapor coatsthe stainless steel surface with titanium and the droplets forms smallround titanium peaks on the coated surface. FIG. 7 is a SEM picture at a52° viewing angle of the 316L stainless steel foil with large amount ofround titanium peaks. The thickness of the Ti coating on stainless steelis 0.5-2 μm. The diameter of the peaks is between 0.5 and 10 microns.The height of the peaks is between 0.5 and 15 microns. The distributiondensity of the peaks is estimated of 1-10 million/cm².

In comparison, the 316L stainless steel foil was also coated withtitanium by DC magnetron sputtering process (SP-Ti-SS). The sputteringprocess is capable of depositing a relatively flat coating on thesubstrate surface. FIG. 10 is a SEM picture of the 316L stainless steelfoil with sputtered titanium on the surface. The finished surfacemaintains the original morphology of the cold rolled stainless steelfoil without large amount of peaks and pits.

Further modification of the titanium coated stainless steel foil surfacewas conducted with hydrofluoric acid etching. The titanium coatedstainless steel foil was soaked in 2 wt % hydrofluoric acid solution for30 seconds (Etched CA-Ti-SS or etched SP-Ti-SS). FIGS. 8 and 9 are SEMpictures of the etched titanium coated stainless steel foil surface at5,000× (for Etched CA-Ti-SS) and 50,000× (for Etched SP-Ti-SS)magnifications, respectively. The etching process produced many pits inthe titanium coating layer, in both round peaks and the flat surface atthe bottom of the peaks. The pits have sharp edges. The diameter of thepits is between 0.5 and 10 microns. The distribution density of the pitsis estimated to be about 10-1,000 million/cm².

The surface property of the titanium coated stainless steel foil wascharacterized with the surface electrical contact resistance (CR) of thestainless steel foil with two pieces of carbon felt (TGP-H-060 TorayPaper) under the compression pressure range of 35-300 psi. Table 2summarizes the results. Table 2 shows that CA-Ti-SS, with the smallround peaks has lower CR than that of relatively flat SP-Ti-SS. EtchedCA-Ti-SS and SP-Ti-SS with the additional pits have a lower TPR thanthat of as coated foils.

TABLE 2 Electrical through plate resistance of titanium coated stainlesssteel foil. Compression Electrical Contact Resistance (mΩ · cm²)Pressure Etched (PSI) SP-Ti-SS CA-Ti-SS Etched CA-Ti-SS SP-Ti-SS 35224.35 32.80 6.20 4.93 51 165.19 23.31 4.27 3.93 91 90.41 12.51 2.292.16 150 52.97 7.49 1.41 1.29 201 37.35 5.60 1.08 0.98 251 27.92 4.470.89 0.79 300 22.29 3.77 0.78 0.70

The etched titanium coated SS surface was superhydrophilic which, hasthe additional advantage for water management in fuel cell stacks. Thelow surface resistance surface modified titanium coated stainless steelin this example can be used as the bipolar plate material for PEM fuelcells.

Example 3

A titanium-tantalum binary alloy with a tantalum concentration of 2%(Ti-2Ta) was prepared. The alloy surface was grinded with #320 sandpaper(grinded Ti-2Ta). Another piece of the #320 sandpaper grinded Ti-2Taalloy was chemically etched in 2 wt % hydrofluoric acid solution for 2minutes (etched Ti-2Ta). It is expected that the surface layercomposition of the grinded Ti-2Ta is the same as the bulk (2% Ta), andthe etched Ti-2Ta has high Ta concentration in the surface layer due tothe faster etching rate of Ti than that of Ta in the condition.Naturally, fresh Ti-2Ta alloy will grow a thin layer of Ta dopedtitanium oxide on the surface. The tantalum dopant concentration in thesurface oxide layer of the etched Ti-2Ta alloy is higher than that ofthe grinded Ti-2Ta alloy. The surface electrical contact resistance ofthe grinded and etched Ti-2Ta alloy was measured with carbon felt(TGP-H-060 Toray Paper) with the compression pressure range of 35-300psi and is summarized in Table 3. Table 3 shows that the etched Ti-2Tahas lower electrical contact resistance than that of the grinded Ti-2Ta,due to the higher Ta concentration in the surface oxide layer.

TABLE 3 Electrical contact resistance of fresh Ti-2Ta with carbon feltCompression Electrical Contact Resistance (mΩ · cm²) Pressure (PSI)Grinded Ti-2Ta Etched Ti-2Ta 35 75.44 31.73 51 62.69 23.26 91 39.6111.82 150 25.44 6.94 201 19.16 5.08 251 15.16 3.95 300 12.64 3.26

The difference of the contact resistance between the grinded and etchedTi-2Ta becomes much bigger with a thicker oxide surface layer. Thegrinded and etched Ti-2Ta alloys were thermally oxidized in air at 300°C. for 1 hour. Their electrical contact resistances were measured withcarbon felt, the same way as the fresh alloy, and is summarized in Table4. Table 4 shows that the contact resistance of carbon felt withthermally oxidized grinded Ti-2Ta is 120.56 mΩ·cm² at 150 psicompression pressure. The contact resistance of carbon felt withthermally oxidized etched Ti-2Ta is 27.09 mΩ·cm² at 150 psi compressionpressure, which is much lower than that of grinded Ti-2Ta.

TABLE 4 Electrical contact resistance of thermally oxidized Ti-2Ta withcarbon felt Compression Electrical Contact Resistance (mΩ · cm²)Pressure (PSI) Grinded Ti-2Ta Etched Ti-2Ta 35 357.43 94.65 51 294.7172.59 91 187.69 43.05 150 120.56 27.09 201 91.51 20.52 251 72.61 16.31300 60.46 13.62

Example 4

A titanium-niobium binary alloy DC magnetron sputtering targets with theniobium concentration of 2% (Ti-2Nb) was prepared. It was used todeposit 5 μm thick Ti-2Nb coating on stainless steel (SS) foil surface(Ti2Nb-SS). The scanning electron microscope (SEM) picture of thesputtered Ti-2Nb on SS surface is shown in FIG. 11 . It includes smallgrains and large grain clusters grown during the sputtering depositionprocess. The coating surface analysis was conducted by Energy-dispersiveX-ray spectroscopy (EDS) and AES. It was found that the bulk of thecoating layer has the same composition as the target material (Ti-2Nb),but the surface layer of the as deposited Ti-2Nb layer has much lowerniobium concentration. It was also found that this surface compositionsegregation was related to the sputtering processing condition. Withdifferent sputtering process conditions, the niobium concentration wascloser to the target material.

A wet chemical etching process with 2 wt % hydrofluoric acid solutionfor 50 seconds was used to remove the niobium depleted titanium alloycoating surface layer (etched Ti2Nb-SS). AES analysis indicated that theniobium concentration (metal base, not include oxygen, nitrogen andother absorbed impurities) of the surface oxide layer was increased to5.7%, the estimated surface oxide thickness is less than 10 nm. Theincrease of niobium concentration in the surface layer was the result ofthe faster etching rate of titanium than niobium. FIGS. 12 and 13 areSEM pictures of the etched Ti2Nb-SS. The surface morphology of theetched Ti2Nb-SS includes many peaks and pits. The peaks have a diameterof 0.05 to 1 μm with an irregular shape. The pits have a width of 0.1 to5 μm. These peaks and pits increase the amount of high pressurecontacting points between the metal plate and other components inelectrochemical devices, which will lead to lower electrical contactresistance.

To further stabilize the surface oxide layer, the SS foil with etchedTi-2Nb coating was thermally oxidized in air at 300° C. for 1 hour togrow a thicker niobium doped titanium oxide layer (oxidized Ti2Nb-SS).AES analysis was conducted on the oxidized surface. It was found thatthe niobium concentration (metal base, not include oxygen, nitrogen andother absorbed impurities) of the surface oxide layer was increased to8.2%, and the surface oxide layer is about 98 nm thick. It is theevident that the thermal oxidization process can further enrich niobiumin the surface oxide layer, produce an anhydrous thick oxide surfacelayer, which is more stable than the hydrated oxide surface layer formedduring etching at ambient temperature. The high temperature process canalso result in a better crystallized doped titanium oxide layer that hashigher electrical conductance.

The through plate resistance of the as etched Ti2Nb-SS and oxidizedTi2Nb-SS was measured with carbon felt (TGP-H-060 Toray Paper) withinthe compression pressure range of 35-300 psi and is summarized in Table5. It was found that the oxidized Ti2Nb-SS has the higher electricalcontact resistance than that of etched Ti2Nb-SS.

The stability of the etched and oxidized Ti2Nb-SS was evaluated by theelectrochemical corrosion tests. The coated stainless steel samples wereimmersed in pH 3 H₂SO₄+0.1 ppm HF solution at 80° C. An electrochemicalpotential of 0.8V_(NHE) was applied on the samples for 100 hours. Then,the through plate resistance of the corroded sample was measured andsummarized in Table 5. It was found that the TPR of the etched Ti2Nb-SSwas smaller than that of oxidized Ti2Nb-SS, but it had a smallelectrical contact resistance increase of etched Ti2Nb-SS after thecorrosion. On the other hand, the oxidized Ti2Nb-SS had no electricalcontact resistance increase after the corrosion, which indicates thesuperior corrosion resistance of the oxidized Ti2Nb-SS.

TABLE 5 Through plate resistance of etched and oxidized Ti2Nb-SS beforeand after the corrosion tests in pH 3 H₂SO₄ + 0.1 ppm HF solution at 80°C. at 0.8 V_(NHE) for 100 hours. Through Plate Resistance (mΩ · cm²)Compression before and after Corrosion Tests Pressure Etched Ti2Nb-SSOxidized Ti2Nb-SS (PSI) Before After Before After 35 4.43 12.38 14.5512.87 51 3.12 9.29 11.53 9.93 91 1.71 5.02 5.86 4.99 150 1.11 3.08 3.272.84 201 0.89 2.37 2.38 2.11 251 0.76 1.95 1.83 1.65 300 0.70 1.70 1.511.38

Example 5

A titanium-niobium binary alloy cathodic arc deposition targets with theniobium concentration of 2% (Ti-2Nb) was prepared. It was used todeposit 2 μm thick Ti-2Nb coating on stainless steel (SS) foil surface(CA Ti2Nb-SS). The scanning electron microscope (SEM) picture of thesputtered Ti-2Nb on SS surface is shown in FIG. 14 . It includes smallpeaks that stand out of the substrate surface. The diameter of the peaksis between 0.05 μm and 10 μm. The height of the peaks is between 0.5 and15 microns. The distribution density of the peaks is estimated of 1-10million/cm². The coating surface analysis was conducted by EDS and XPS.It was found that the bulk and the surface layer of the coating layerhave the same composition (˜2% Nb in Ti) as the target material(Ti-2Nb).

The CA Ti2Nb-SS surface was further modified by wet chemical etching in2 wt % hydrofluoric acid solution for 30 seconds (etched CA Ti2Nb-SS).FIG. 15 is the SEM picture of the etched CA Ti2Nb-SS. It shows that theetching step has created many pits on the surface. These pits have asharp edge and the diameter of the holes is between 1 to 10 μm. Inaddition, the etched surface does not show the grain boundary of thecoating material, which indicates that the coating deposited by thecathodic arc was in amorphous structure. The surface oxide layercomposition of etched CA Ti2Nb-SS was analyzed by AES. It was found thatthe niobium concentration (metal base) in the surface oxide layer wasenriched to 6.3% by the etching.

The peaks and pits on the surface will effectively increase the amountof the high pressure contacting points of the metal plates with othercomponents, thus reducing the electrical contact resistance. The throughplate resistance of the as etched Ti2Nb-SS and oxidized Ti2Nb-SS wasmeasured with carbon felt (TGP-H-060 Toray Paper) within the compressionpressure range of 35-300 psi and is summarized in Table 6. It was foundthat the both CA Ti2Nb-SS and etched CA Ti2Nb-SS have low TPR. Theetched plate has lower resistance due to the higher niobiumconcentration in the doped titanium oxide surface and the additionalhigh pressure contact points created by the pits with the sharp edges.

TABLE 6 Through plate resistance of as deposited and etched CA Ti2Nb-SSCompression Through Plate Resistance (mΩ · cm²) Pressure (PSI) CATi2Nb-SS Etched CA Ti2Nb-SS 35 18.36 4.12 51 12.97 3.27 91 6.95 2.08 1504.17 1.48 201 3.15 1.25 251 2.55 1.11 300 2.19 1.02

Example 6

The titanium plate surface is blasted with sand to create small peaksand pits on the surface. The sand has the average particle size between22 and 0.1 microns (#400 and finer). The finished surface has peaks withthe dimension (width) of 0.01-1 microns, and the pits with the dimension(width and depth) of 1-20 microns.

The sand blasted titanium plate surface is coated with a thin layer of aTi-2Nb alloy by a PVD process. The thickness of the Ti-2Nb coating is0.005-5 μm. A semi-conductive niobium doped titanium oxide surface layeris formed naturally on the coated titanium plate. An etching step can beapplied to modify the composition of surface oxide layer to furtherimprove the surface electrical conductance of the doped titanium oxidelayer with higher concentrations of the dopant element, such as niobium.

The coated titanium alloy can be used in fuel cells, electrolyzers andcapacitors to reduce their internal electrical resistance. Depending onthe application requirement, the thickness of the niobium doped titaniumoxide surface layer can be increased by an oxidization process. Theoxidization process can be plasma oxidizing, anodizing (electricaloxidizing), chemical oxidizing in solutions with oxidizing agents andthermal oxidizing at elevated temperature in oxygen containingatmosphere.

The preferred oxidizing method is thermal oxidization in air. Thetemperature is between 100-600° C. The atmosphere could also be atdifferent pressure (from high pressure to vacuum) and with other oxygencontaining compounds, such as steam and carbon dioxide for betterquality of the surface oxide layer.

Example 7

A titanium-niobium binary alloy DC magnetron sputtering targets with theniobium concentration of 2% (Ti-2Nb) was prepared. It was used todeposit a 5 μm thick Ti-2Nb coating on a stainless steel (SS) foilsurface. The Ti-2Nb coated SS was then placed in a PE-200RIE reactiveplasma etching system from Plasma Etch, Inc. The etching condition was100 W for 30 minutes, with an O₂ flow rate of 15 mil/min and a CF₄ flowrate of 85 ml/min. The vacuum was at about 0.2 torr. The through plateresistance of the as etched Ti-2Nb coated SS was measured with carbonfelt (TGP-H-060 Toray Paper). It was found that the through plateresistance of reactive plasma etched Ti-2Nb coated stainless steel wasreduced to 7.58 mΩ·cm² at 150 psi compression pressure. The AES surfaceanalysis found the niobium concentration in the surface oxide layer wasenriched to 18%.

Example 8

A titanium-niobium binary alloy DC magnetron sputtering targets with theniobium concentration of 2% (Ti-2Nb) is prepared. It was used to deposit5 μm thick Ti-2Nb coating on a stainless steel (SS) foil surface. TheTi-2Nb coated SS was put in an aluminum vacuum chamber with XeF₂particles. The foil was held in a vacuum for 60 minutes. 200 ml/minargon gas flow was maintained during the process to keep the vacuum at0.45 torr. The through plate resistance of the as etched Ti-2Nb coatedSS was measured with carbon felt (TGP-H-060 Toray Paper). It was foundthat the through plate resistance of reactive plasma etched Ti-2Nbcoated stainless steel was reduced to 17.40 mΩ·cm² at 150 psicompression pressure.

Example 9

A titanium-niobium binary alloy DC magnetron sputtering targets with theniobium concentration of 2% (Ti-2Nb) is prepared. It was used to deposit5 μm thick Ti-2Nb coating on a stainless steel (SS) foil surface. TheTi-2Nb coated SS was put in PE-200RIE reactive plasma etching system tobe etched at different conditions. Table 7 summarized the etchingconditions with the oxygen gas flow of 15 cc/min and CF4 gas flow of 85cc/min at vacuum of 0.2 torr, and the final Nb concentration in thesurface oxide layer after etched. It shows that the Nb concentration inthe surface oxide layer is dramatically increased. The finalconcentration is related with the plasma etching power and etching time.At high etching power and long etching time, more fluorine radicals aregenerated and the Nb concentration enrichment is more significant in thesurface oxide layer of the etched plate. This experiment indicates thatthe surface composition can be tailored by the etching condition. It wasalso found in another experiment that plasma etching at low power andshort time will not enrich niobium in the surface oxide layer.

TABLE 7 Ti2Nb-SS Plasima Etching Conditions and Nb Concentration inSurface Layer. Sample # Etching Conditions Surface layer NbConcentration 34-74-1  50 W, 60 min 28.5% 34-75-1 250 W, 60 min 83.4%34-75-3 250 W, 10 min 49.6%

Example 10

A Ti—Nb alloy sheet with 3% of Nb is prepared vacuum melting and coldrolling. Then, the sheet is polished to obtain a smooth surface. Twopieces of samples are cut from the sheet. One piece is oxidized in airat 400° C. for one hour, the other piece is oxidized in air at 600° C.for 1 hour. XPS analysis was conducted on these samples. It was foundthat the Nb concentration in surface oxide layer of both samples wasincreased after the thermal oxidization. The Nb concentration of the400° C. oxidized sample surface is 5.3%, and the 600° C. oxidized sampleis 8.5%. This experiment indicates the dopant (Nb) concentration in thesurface oxide layer can be controlled by the heat treatment conditions.The higher treatment temperature will result in the high Nbconcentration in the oxide layer.

Example 11

A titanium-niobium binary alloy cathodic arc deposition targets with theniobium concentration of 7% (Ti-7Nb) is prepared. It is used to deposita 2 μm thick Ti-7Nb coating on a stainless steel (SS) foil surface (CATi-7Nb-SS). With the proper process control, the Ti-7Nb coating layer isin single amorphous phase, with many round shape peaks on the surface.This coating can reduce the surface electrical contact resistance of themetal foil. And its resistance can be further reduced by a surfaceetching process using 2 wt % hydrofluoric acid solutions to create smallpits on the surface, in addition to small peaks deposited by cathodicarc deposition. The niobium concentration in the surface oxide layer isalso increase after the etching.

Example 12

A titanium-niobium binary alloy DC magnetron sputtering targets with theniobium concentration of 1% (Ti-1Nb) is prepared. It is used to deposita 0.5 μm thick Ti-1Nb coating on a stainless steel (SS) foil surface.Before the coated stainless steel is exposed to air, after Ti-1Nbsputtering, the coated stainless steel is moved to another vacuumchamber. The vacuum is controlled at 0.01 mbar with chlorine and argongas mixture flow of 200 ml/min for 2 minutes. The gaseous vacuum etchingcan enrich the niobium concentration on the Ti-1Nb coating surface toincrease the surface electrical conductance of the coated stainlesssteel.

Example 13

A stainless steel surface is coated with a thin layer of titanium bye-beam evaporation. The finished surface has peaks with the dimension(width) of 0.01-1 microns, and the valley with the dimension (width anddepth) of 1-20 microns.

A Ti-2Ta alloy coating is applied on the titanium coated stainless steelsurface to obtain the semiconductive tantalum doped titanium oxidesurface oxide layer. The thickness of the Ti-2Ta layer is between 0.005and 0.1 microns.

The properties of the tantalum doped titanium oxide layer can beoptimized by surface etching (to increase tantalum concentration in thedoped oxide surface layer) and thermal oxidization (to stabilize thedoped oxide surface layer).

Example 14

An aluminum foil is used as the substrate material for the currentcollector in Li ion batteries and capacitors. The foil is first coldrolled with a roller that has special surface texture. The texture canemboss the aluminum surface to the morphology that includes many smallpeaks and pits. The peaks have the dimension (width) of 0.1-1 microns,and the pits have the dimension (width and depth) of 1-20 microns

A Ti-2Ta alloy coating is applied on the aluminum surface to obtain thesemiconductive tantalum doped titanium oxide surface oxide layer. Thethickness of the Ti-2Ta layer is between 0.005 and 0.1 microns.

The properties of the tantalum doped titanium oxide layer can beoptimized by surface etching (to increase tantalum concentration in thedoped oxide surface layer) and thermal oxidization (to stabilize thedoped oxide surface layer).

Example 15

A niobium-titanium binary alloy DC magnetron sputtering targets with thetitanium concentration of 2% (Nb-2Ti) is prepared. It was used todeposit a 2 μm thick Ti-2Nb coating on a stainless steel (SS) foilsurface. The coated stainless steel surface is then modified with 30%sodium hydroxide+10% hydrogen peroxide mixture solution at roomtemperature. The solution will slowly etch the niobium alloy coating tomodify the surface oxide layer composition and create small pits on thesurface for low surface electrical contact resistance.

Example 16

A porous Ti disk with proper pore size is selected as the gas diffusionlayer of a water electrolyzer. The diameter of the disk is 8″. The diskis soaked in 500 ml 1.5 wt % hydrofluoric acid solution for 60 minutesfor surface etching. This etching process creates small titanium peakson surface. These peaks decrease the electrical contact resistance ofthe gas diffusion layer with electrode catalyst and the bipolar plate ofthe electrolyzer cells.

Example 17

A titanium-niobium alloy powder with the niobium concentration of 1%(Ti-1Nb) is used to make porous Ti-1Nb porous Ti-1Nb plates as the gasdiffusion layer for PEM electrolyzers. After the high temperature sinterstep to build solid porous structures, the plate is etched in 2 wt %hydrofluoric acid solution for 30 seconds. The etching step enriches theniobium concentration on the surface of the porous Ti-1Nb plate to 7-9%.Then, the oxide surface layer with the high niobium concentration isgrown on the plate surface, for the low surface contact resistance withelectrode catalyst and bipolar plates.

Example 18

A 316L stainless steel plate is used as the substrate. The plate surfaceis rapidly melted and recrystallized by high intensity laser scanning onthe surface. This melt/recrystallization process will reduce the grainsize of stainless steel from typical 5-10 μm to less than 5 μm. Then,the plate is immersed in stainless steel etchant solution for 15seconds. The etchant solution contains 15% ferric chloride, 3% copperammonium chloride, 25% hydrochloric acid in distilled water. The etchedplate has a micro-textured surface structure. The stand-outs on thesurface are the same size as the fine grain (<5 μm) after the surfacerecrystallization. Then, the micro-textured stainless plate is coatedwith 10 nm thick Ti by sputtering process. The Ti coated, micro-texturedstainless steel will have low surface electrical contact resistance.

Example 19

A titanium plate is used as the substrate. The plate surface isdeposited with a layer of Ti particles with the particle size between0.5-5 μm on the plate surface. The particle density on the surface isbetween 50-500 million particles/cm². Then, the surface is rapidlyscanned by high intensity layer that instantly melts the particles. Thescan time is very short that the substrate surface is not or notsignificantly melted. Then the melted Ti particles are cooled and bondedon the Ti plate surface in the form of round peaks with the distributiondensity of 50-500 million peaks/cm². These peaks perform as highpressure contacting points with other components to reduce the surfaceelectrical contact resistance.

Example 20

In this example, stainless steel is used as the substrate. A thermalspray process is used to deposit small stainless steel powder to form amicro-textured stainless steel having small stainless steel peaks on thesubstrate surface. The particle size of the stainless steel powder is0.5-5 μm. The spray process is controlled to having low velocity and arelatively cold condition of the melted stainless particles that avoidflat splats deposited on the substrate surface. The finished peaks havethe diameter of 1-10 μm and the high of 0.2-5 μm. Then, themicro-textured stainless steel is coated with 50 nm thick of Ti-2Nballoy by DC magnetron sputtering process. The surface oxide layer of theTi-2Nb alloy will protect the stainless steel from corrosion andmaintain a low surface electrical contact resistance.

Example 21

In this example, a titanium plate is used as the substrate. A thermalspray process is used to deposit and bond Ti-2Nb alloy powders on thesubstrate surface. Then, coated Ti plate is chemically etched in 3% HFsolution for 120 seconds to remove the thick surface oxide layer formedin thermal spray process, and increase the Nb concentration in thesurface oxide layer of the etched plate. The etched plate is then heattreated at 300° C. for 30 minutes in air. A dense, anhydrous surfaceoxide is formed by the thermal oxidization, which is more corrosionresistive than the surface oxide layer formed in ambient temperature.The finished Ti plates have higher (3-15%) niobium concentration (metalbase) in the surface oxide layer, which results in the low surfaceelectrical resistance and high corrosion resistance for electrolyzerapplications.

As mentioned above, it should be appreciated that embodiments disclosedherein are not limited to the specific features discussed above. Forexample, the growing conditions listed herein are mere examples and itshould be appreciated that the growing can occur before the substrate isplaced within the device or after. The exact process used (i.e., thermaloxidation, anodizing, plasma oxidation) will depend on the type ofapplication or processing costs.

The foregoing examples are provided merely for the purpose ofexplanation and are in no way to be construed as limiting. Whilereference to various embodiments is made, the words used herein arewords of description and illustration, rather than words of limitation.Further, although reference to particular means, materials, andembodiments are shown, there is no limitation to the particularsdisclosed herein. Rather, the embodiments extend to all functionallyequivalent structures, methods, and uses, such as are within the scopeof the appended claims.

What is claims is:
 1. A metallic component having a surface with lowelectrical contact resistance, said component comprising: a metallicsubstrate having a titanium alloy surface containing high valanceelements that forms a surface oxide, wherein the weighted averagevalance of metallic elements in the surface oxide are in the range of+4.02 to +4.25, the high valance element concentration in the surfaceoxide layer is higher than that in a bulk of the titanium alloy.
 2. Ametallic component having a surface with low electrical contactresistance, said component comprising: a metallic substrate having atitanium alloy surface containing niobium or tantalum, wherein theniobium or tantalum concentration in the surface oxide layer is higherthan that in a bulk of the titanium alloy, wherein a concentration ofniobium or tantalum in the titanium alloy is between about 0.01% and 3%.3. A metallic component having a surface with low electrical contactresistance, said component comprising: a metallic substrate having atitanium alloy surface containing niobium or tantalum, wherein theniobium or tantalum concentration in the surface oxide layer is higherthan that in a bulk of the titanium alloy, wherein a concentration ofniobium or tantalum in the surface oxide layer is between about 4% and15%.
 4. The metallic component of claim 1 further comprising a titaniumalloy substrate containing other alloy elements with the weightedvalance of all metallic elements in surface oxide layer higher than 4.5. The metallic component of claim 1 further comprising a metalsubstrate having a titanium alloy coating on a surface of the substrate,said coating containing niobium or tantalum.
 6. The metallic componentof claim 5, wherein the titanium alloy coating is provided by one ofthermal spraying or physical vapor deposition.
 7. A metallic componenthaving a surface with low electrical contact resistance, said componentcomprising: a metallic substrate having a titanium alloy surfacecontaining niobium or tantalum, wherein the niobium or tantalumconcentration in the surface oxide layer is higher than that in a bulkof the titanium alloy, wherein the high concentration of niobium ortantalum in the surface oxide layer is obtained by: chemically etching asurface of the titanium alloy to remove the original surface layer; andnaturally forming a thin oxide surface layer on the etched titaniumalloy.
 8. The metallic component of claim 7, wherein the chemicallyetching process comprises immersing the titanium alloy surface insolutions containing hydrofluoric acid.
 9. The metallic component ofclaim 7, wherein the chemically etching process is performed in a lowpressure environment having fluorine or chlorine containing chemicals orradicals in vapor phase.
 10. A metallic component having a surface withlow electrical contact resistance, said component comprising: a metallicsubstrate having a titanium alloy surface containing niobium ortantalum, wherein the niobium or tantalum concentration in the surfaceoxide layer is higher than that in a bulk of the titanium alloy, whereinthe high concentration of niobium or tantalum in the surface oxide layeris obtained by a thermal oxidization of the titanium alloy surface. 11.The metallic component of claim 10, wherein the thermal oxidizationtemperature is between 100-800° C.
 12. A metallic component having asurface with low electrical contact resistance, said componentcomprising: a metallic substrate having a titanium alloy surfacecontaining niobium or tantalum, wherein the niobium or tantalumconcentration in the surface oxide layer is higher than that in a bulkof the titanium alloy, wherein said component comprises pure titaniumplate as the substrate; a coating of titanium alloy is deposited on thepure titanium substrate surface by thermal spray, the sprayed plate ischemically etched to remove the original surface oxide layer andincrease the concentration of alloy element in the new surface oxidelayer.
 13. The component in claim 12, titanium alloy contains niobium ortantalum, with the concentration between 0.01-3%.
 14. The component inclaim 12, niobium or tantalum concentration in the surface oxide layeris between 3-15%.
 15. The component of claim 12, the surface oxide layeron the etched titanium alloy surface is further modified by thermaloxidization at temperature to obtain the dense, anhydrous and niobium ortantalum enriched surface oxide layer.
 16. The metallic component ofclaim 15, wherein the thermal oxidization temperature is between100-800° C.